Co-pending patent application entitled "Prosthetic Spinal Disc Nucleus" and "Method for Surgical Implantation Of A Prosthetic Spinal Disc Nucleus" were filed on the same day as the present application and are assigned to the same assignee.
The present invention concerns a surgical tool for piercing an anulus of a human intervertebral disc. More particularly, it relates to an spinal anulus cutter for cutting a flap into the anulus for any reason, for example as part of a discectomy surgery or prior to surgical removal of a spinal discal nucleus, e.g. for herniated discs or implantation of a prosthetic spinal disc nucleus body.
The vertebrate spine is the axis of the skeleton on which all of the body parts "hang". In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, in turn supported by the hip and leg bones. The bony vertebral bones of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation.
The typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The centra of adjacent vertebrae are supported by intervertebral discs. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch which extends posteriorly and acts to protect the spinal cord's posterior side is known as the lamina. Projecting from the posterior region of the neural arch is a midline spinous process.
The intervertebral disc primarily serves as a mechanical cushion between vertebral bones, permitting controlled motion within vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulpous ("nucleus"), the anulus fibrosus ("anulus") and two vertebral end plates. The two vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus act to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring which binds together adjacent vertebrae. This fibrous portion, which is much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fiber layers of the anulus consist of fifteen to twenty overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 40 degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotates in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleus. The healthy nucleus is a largely gel-like substance having a high water content, and like air in a tire, serves to keep the anulus tight yet flexible. The nucleus-gel moves slightly within the anulus when force is exerted on the adjacent vertebrae while bending, lifting, etc.
The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anulus confines. The mass of a herniated or "slipped" nucleus tissue can compress a spinal nerve, resulting in leg pain or loss of muscle control and even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
To alleviate pain from a herniation, it is usually necessary for the surgeon to access the nucleus area within the anulus. This may be to perform discectomy surgery, remove the herniated anulus or nucleus, or implant a prosthetic nucleus device. To access the nucleus, the surgeon is required to cut through and/or remove at least a portion of the anulus. A surgical concern, not previously addressed, is the potential damage imparted upon the anulus during this surgery.
As previously stated, the normal anular plies act to keep the anulus tight about the nucleus. During the aforementioned surgeries, a surgical knife or tool is used to completely sever some portion of the anulus and/or remove an entire section or a "plug" of the anulus tissue. When an entire section of the anulus is cut or removed, the layers making up the anulus "fray" and/or "pull back" and the constraining or tightening ability of that portion of the anulus is lost. Further, the chances of the anulus healing with restoration of full strength are greatly diminished, while the likelihood of nucleus reherniation increases. An even greater concern arises where a significant portion of the anulus is removed entirely. These problems present themselves when the anulus is incised or removed during any type of discectomy surgery. Further, the same concerns arise whenever a surgeon is required to cut through an encapsulating ligament (such as the anulus, knee, shoulder, etc.) to access the space or material surrounded by the ligament. A more desirable solution is to leave the ligament or anulus at least partially intact during and after surgery.
Preserving the integrity of the anulus enhances the potential for physical healing in the disc area. As part of most disc related surgeries, a surgical tool must be employed to pierce the anulus to provide access to the area. Therefore, a substantial need exists for a surgical tool having the ability to pierce the anulus in such a way as to provide access to the discal area but allow for regenerative recovery of the anular fibers afterwards, and prevent the damaging or destruction of the tightening or constraining ability of the anulus itself. Such a tool will have similar applicability to any encapsulating ligamentary area.